To facilitate studies of multicomponent protein complexes, I have developed an Escherichia coil expression system which coexpresses up to four polypeptides from a single plasmid, The modular nature of the system enables efficient subcloning of a gene into each of the 4 cassettes in the polycistronic expression vector. Restriction sites present in the polycistronic expression vector allow both affinity tagged and untagged complexes to be overexpressed, I demonstrate successful use of the expression system for binary and ternary complexes, including the reconstitution of the VI-TG elonginC-elonginB complex in E. coli and purification of the complex by affinity and ion-exchange chromatography, This polycistronic expression system should provide an important alternative to in vitro reconstitution of multicomponent complexes.